The present invention relates generally to filters, and is directed more particularly to a filter which filters both solid particulates and water from air when the air passes through the filter in one direction, and which releases water to air when the air passes through the filter in a second direction, thereby regenerating the water-removing capability of the filter.
In a typical hydraulic power system, hydraulic fluid is stored in a reservoir. The fluid level in the reservoir rises and falls as the fluid is used in the system. When the fluid level falls in the reservoir, a vacuum is created, and ambient air enters the reservoir through a filtered vent. It is essential that water and dust particles be removed or filtered from the incoming air, as water and dust contaminate the fluid and cause abrasive wear and corrosion of the system components. When the fluid level rises in the reservoir, the air in the chamber is pumped out of the vent.
Simple filters which remove only airborne particles are well known in the art. These filters usually comprise a housing having an inlet port and an outlet port, and a filter element disposed between the respective ports to remove solid particles.
Filters which remove moisture only are also known. These filters typically use some sort of desiccant to remove the moisture from an air stream. For example, U.S. Pat. No. 3,303,621 (Hill) discloses a gas drier which uses a deliquescent desiccant to dry air. However, in filters of this nature, the desiccant has a finite life and must be replaced periodically.
Filters which remove both airborne particulates and moisture are also known in the art. U.S. Pat. No. 4,504,289 and its continuation U.S. Pat. No. 4,548,624 (Waller) disclose a hygroscopic breather cap which contains two knitted fabric filters to remove dust particles from air entering a petroleum tank, and also contains a solid desiccant to remove moisture. In this filter the air passes through the filter in one direction, and the desiccant has a finite capacity. When the desiccant becomes saturated, it must be replaced.
What is needed, then, is a filter which removes airborne contaminants and moisture from air when air passes through the filter in a first direction, and yet regenerates its capacity to filter moisture when air passes through the filter in a second direction.